This invention relates to aqueous organic polyisocyanate adhesives containing a major portion of the lignin (waste liquor solids) resulting from the chemical pulping of lignocellulose. This lignin product comprises principally recovered lignin which has been modified by the chemical pulping procedure to which it has been subjected and accordingly is to be distinguished from native lignin, i.e. the "proto-lignin" found in the lignocellulosic structure of growing plants.
Organic polyisocyanate resins containing lignin are not new to the art.
Nichols U.S. Pat. No. 2,854,422 discloses a composition comprising diisocyanate-linked elastomers and lignin wherein the lignin is added to a formed polyurethane and is merely present as a filler.
Mills et al U.S. Pat. No. 2,906,718 discloses lignin-reinforceable, vulcanizeable rubber stocks wherein the lignin takes the place of the usual carbon black reinforcing material.
Moorer et al U.S. Pat. No. 3,519,581 disclose a method of producing synthetic lignin-polyisocyanate resins, which may be used as adhesives, by dissolving lignin in a glycol solvent and reacting it with an organic polyisocyanate.
Santelli et al U.S. Pat. No. 3,577,358 disclose organic isocyanate-lignin reaction products, which also may be used as adhesives, formed by reacting an organic polyisocyanate with lignin dissolved in a non-aqueous solvent therefor, the solvent again being a glycol, or dioxane.
However, the polyisocyanate-lignin adhesives of the prior art are neither designed nor suitable for large scale use as bonding agents for wood products, in particular for bonding wood particles or fibers together in the manufacture of wood composition board. The reason for this is to be found in the fact that the patentees sought assiduously to avoid the inclusion of water in the formulation of the adhesives of their inventions in order to avoid excessive urethane formation and foaming during their preparation and use. Moorer et al state in U.S. Pat. No. 3,519,581 for example, at column 6, lines 21-28:
"Water is a solvent for the alkali metal and the ammonium salts of lignin. However, water is so extremely reactive with polyisocyanates that water is not availed of as a solvent for the lignin, but when used is only used in small amount for reaction with part of the polyisocyanate for the production of carbon dioxide as a by-product, the generation of the carbon dioxide resulting in the production of a foam."
The prior art accordingly teaches the use of water with polyisocyanate-lignin mixtures only as a terminal foaming or blowing agent when a foamed urethane product is desired. The prior art adhesives incorporate not an aqueous solvent for the lignin, but rather a proportion of a glycol, ether, or other non-aqueous solvent. They accordingly are totally unsuited for use as wood bonding adhesives.
Wood adhesives must be tolerant of water first, because of the inherent water content of pulping waste liquors, which are the primary sources of the lignin solids used in the adhesives, and second because the wood pieces to be bonded usually have an appreciable water content. For this reason it is impractical, indeed impossible, to employ water-sensitive polyisocyanates as components of adhesive mixtures containing also aqueous solutions of lignin, the adhesive mixtures to be used in the gluing together of pieces of wet wood.
Furthermore, in the art of gluing wood, the adhesives to be used in gluing wood must be of relatively low viscosity so that they may be applied to the wood pieces by such standard techniques as spray-, roll-, or curtain-coating. When thus applied, they must cover the pieces uniformly and bond them together efficiently.
This desired result cannot be achieved by the polyisocyanate-lignin adhesive of the prior art. The prior art adhesives are gummy products of extremely high viscosity. For example, the polyisocyanate-lignin adhesive disclosed in Santelli et al U.S. Pat. No. 3,577,358, at Col. 8, lines 16-30 is described as a paste having a softening point of over 270.degree. C. This paste is disclosed as an adhesive for glass. It is totally unsuited as an adhesive for wood. Its high viscosity prevents its effective application.
I now have discovered, and it is the essence of the present invention, that under certain circumstances water in fact may be used in major amount in the manufacture of organic polyisocyanate-lignin adhesives to be used in the manufacture of bonded wood products, in particular in the manufacture of wood composition board products such as wood particle board and wood fiber board. This discovery is based on three principal observations.
First, water is an excellent solvent for lignin and may be used in that capacity in the formulation of the herein described adhesive.
Second, the polyisocyanates in the molecular weight range of 200-10,000 are oily, strongly hydrophobic liquids which at ambient temperature react only very slowly with water to form urethanes and carbon dioxide gas. This reaction occurs only at the limited interface between the oily polyisocyanate phase and the water phase. Even when the polyisocyanate is emulsified with water, the reaction takes place so slowly at ordinary mill temperatures that gas formation is not a problem. The aqueous solution of lignin provided by ordinary waste liquor resulting from the chemical pulping of lignocellulose thus may be used as the water solution of lignin without any modification whatsoever.
Third, the gaseous evolution which in minor degree does take place in the gluing of wood pieces with the herein described adhesive is entirely accommodated by the porous matrix to which the glue is applied. In particular, a matrix comprising a wood composition board furnish is sufficiently porous to absorb totally any carbon dioxide gas evolved by the reaction between the polyisocyanates and water without adversely affecting product properties.
In its broad aspect the adhesive of the invention accordingly comprises an organic polyisocyanate having a molecular weight in the range of from 200 to 10,000 and an aqueous solution of waste liquor solids, preferably the waste liquor derived from chemical pulping of lignocellulose, the water being used in amount sufficient to dissolve the lignin and to establish a working adhesive viscosity.
The invention further comprises as a new article of manufacture the glued product comprising pieces of wood bonded together with the aforesaid adhesive. In particular, it comprises wood composition board comprising a hot-consolidated mat comprising small pieces of wood glued together with the said adhesive.